speed of light

American  

[speed uhv lahyt] / ˈspid əv ˈlaɪt /

noun

1. Physics, Optics. a fundamental universal constant, the speed at which light and all forms of electromagnetic radiation travel in a vacuum, standardized as 186,282.4 miles per second (299,792,458 meters per second).

   The speed of light, often represented by the letter c, figures prominently in modern physics, as in Einstein’s famous equation E = mc2, which expresses the relation between mass (m) and energy (E).

2. an extremely fast rate.

   They gobbled those appetizers up at the speed of light.

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Etymology

Origin of speed of light

First recorded in 1820–25

Example Sentences

The team also measured the jets' speed, finding that they travel at roughly half the speed of light, or about 150,000 kilometers per second.

From Science Daily • Apr. 18, 2026

Now, the latest artificial-intelligence bottleneck is optical interconnects, or the high-speed systems that allow massive chip clusters to communicate at the speed of light.

From MarketWatch • Mar. 7, 2026

Grant’s work is delicate: He cleaves each strand of a fiber-optic cable for a clean edge, then uses a machine to fuse the hair-thin filaments, which carry digital data at the speed of light.

From The Wall Street Journal • Feb. 1, 2026

"If software and AI models move at the speed of light, energy and hardware move at the speed of physics."

From Barron's • Jan. 3, 2026

Similarly, the past of P can be defined as the set of all events from which it is possible to reach the event P traveling at or below the speed of light.

From "A Brief History of Time: And Other Essays" by Stephen Hawking